Cellular digital mobile radio system and method of transmitting information in a digital cellular mobile radio system

ABSTRACT

The invention relates to a cellular digital mobile radio system including base stations (B m , B n ) and mobile stations (MS 1 , MS 2 ) with transmitters and receivers. The invention also relates to a method of transmitting message information digitally between mobile and base stations in such a system. In accordance with the invention, at least two base station transmitters (B ma , B mb , P na , B nb ) at a given transmitting distance from each other are assigned to each of certain cells (C m , C n ) within a restricted geographical area. The base station transmitters which are assigned to the same cell transmit digitally modulated radio signals within the same frequency range at least partially simultaneously to the mobile stations of the cell. The radio signals from different base station transmitters associated with the same cell are digitally modulated with the same message information to the mobile stations in the cell. Different base station transmitters ( 9 A,  9 B) preferably transmit the digitally modulated radio signals with the same message information to a given mobile station with a given mutual transmission time shift. Here, the transmission time shift is selected individually for each mobile station, such that corresponding digitally modulated radio signals with the same message information to a given mobile station from different base station transmitters arrive practically simultaneously at the mobile station.

This application is a continuation of application Ser. No. 08/136,760,filed Oct. 15, 1993 now U.S. Pat. No. Re. 36,017, which is acontinuation of application Ser. No. 315,561, filed Feb. 27, 1989, nowabandoned.

TECHNICAL FIELD

The present invention relates to mobile radio systems. More specificallythe invention relates to a digital, cellular, mobile radio system. Theinvention also relates to a method of transmitting information digitallyto and from mobile stations in a cellular mobile radio system.

BACKGROUND ART

The mobile radio systems that were first taken into common use were ofanalog type, i.e. message information was transmitted in analog form toand from mobile stations by transmitting and receiving analog-modulatedradio signals. In such systems it is known to have two or more basestation transmitters at a distance from each other simultaneouslytransmitting radio signals within the same frequency range and modulatedwith the same message information to the mobile stations. Such mobileradio systems are described in EP 0040731 and EP 0072479, as well as inthe two publications: NTG-Fachberichte, Bewegliche Funkdienste, Vorträgeder NTG-Fachtagung vom 25. bis Nov. 27, 1985 in Munich,“GLEICHKANALFUNKSYSTEME FüR DIE FREQUENZÖKONISCHE VERSORGUNG GROSSERGEBIETE” Berndt Heynisch pp 41-46, VDE-VERLAG GmbH, Berlin,Elektrizitätswirtschaft, Jg. 80(1981), Heft 6, pp 187-198“Quasissynchroner Gleichwellenfunk-ein Gleichkanalfunk-Ver-fahren zurErhöhung der Erreichbarkeit in Mobilfunknetzen”.

In known systems of the kind in question here, it is known to transmitmessage information from a central station or exchange to the basestation transmitters via either cables or radio signals. It is alsoknown to have equalizers in the fixed part of the mobile radio systemfor equalizing differences in propagation time and attenuation intransmission from the exchange to the base station transmitters. Theequalizers can be at the exchange and/or at the base stationtransmitters. The object of the equalisers is that irrespective ofposition in relation to the exchange the base station transmitters shalltransmit the radio signals simultaneously, and modulated with the samemessage information.

Digital mobile radio systems in which message information is transmitteddigitally to and from mobile stations by transmission and reception ofdigitally modulated signals have been proposed in U.S. Pat. No.4,675,863 and “Digital Mobile Telephone System Using TD/FDMA Scheme”,Kota Kinoshita, Masaharu Hata and Kenkichi Hirade, IEEE TRANSACTIONS ONVEHICULAR TECHNOLOGY, VOL. VT-31, NO. 4, NOVEMBER 1982, pp 153-157.

It has been proposed to have adaptive equalizers in mobile radiostations in digital radio systems, whereby multi-path propagation ofradio signals can be used to improve signal quality, instead of themultipath propagation acting as noise. Among the publications onadaptive equalizers in digital mobile radio systems can be mentioned:“Multi-path Equalization for Digital Cellular Radio Operation at 300 k.bits/s”. K Raith, J-E Sjternvall and J Uddenfeldt, 36th IEEE VehicularTechnology Conference, pp 268-272, Dallas, Tex., U.S.A. May 1986. “RadioTest Performance of a Narrowband TDMA System”, J-E Stjernvall, B.Hedberg, and S Ekmark, IEEE Vehicular Conference. Tampa, Fla., U.S.A.,June 1987, RADIO TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-DMS 90,J-E Stjervall, B. Hedberg, K Raith, T Backstrom and R Lofdahl.

SUMMARY OF THE INVENTION

In mobile radio systems there are problems due to reflections and radioshadows from natural obstacles such as rocks and hills, as well asstructures such as buildings. These problems are especially troublesomein transmitting information requiring great accessibility/reliabilityand high transmission speed. In particular the problems may become largein certain urban environments where the propagation conditions for radiosignals can vary heavily within a small geographic area, while radiotraffic is intensive at the same time. Up to now attempts have been madeto solve these problems by having adaptive equalizers in the mobilestations and small cells with specially selected positioning of the basestation transmitters. In areas with much traffic it is, however, adesire to be able to select the size of the cells and their positions inthe mobile radio system cell plan in an optimum way with respect to thetraffic handling capacity of the system. Reducing the cell size andselecting the positions of the small cells to avoid radio shadows thusinvolves a complication. Another complication resulting from thereduction of cell size to below what is necessary for reasons ofcapacity is that the number of handovers increases.

The object of the present invention is to solve the above-mentionedproblems and complications, and to provide a method and a cellulardigital mobile radio system which are also suitable for transmittinginformation requiring great accessibility/reliability and hightransmission speed.

What is distinguishing for a method and a digital cellular mobile radiosystem in accordance with the present invention, and particularlypreferred embodiments thereof is disclosed in the independent anddependent claims. Somewhat simplified, it may be said that according tothe present invention at least two base station transmitters areutilized for each of a plurality of cells, these transmitters being at adistance from each other and at least partially simultaneouslytransmitting radio signals within the same frequency range digitallymodulated with the same message information to the mobile stations inthe cell. The digital modulation is changed with a modulation timeinterval which is adapted to the greatest transmitting distance betweentwo base station transmitters serving the same cell in an area. Themobile stations have adaptive equalizers for reconstructing the digitalmodulation in the transmitted signals from the signals received during areception time interval, which is also adapted to the greatesttransmitting distance between two base station transmitters serving thesame cell in an area.

In a preferred embodiment of a method in accordance with the presentinvention, the digitally modulated signals are transmitted with the samemessage information to a given mobile station with a given, mutualtransmission time shift from the different base stations. Thetransmission shift is then selected such that it counteracts thedifference in arrival time for the signals from the different basestation transmitters to this mobile station.

In each mobile station, there is preferably estimated the arrival timeshift between the digitally modulated radio signals with the samemessage information from the different base station transmitters.Information about the estimated arrival time shift at the respectivemobile station is transmitted from there to at least one base stationtransmitter. This estimated arrival time shift is utilized at the basestation for selecting the transmission time shift for at least one basestation transmitter. A transmission time shift is thus obtainedindividually for each affected mobile station, and which is adjusted forthis particular mobile stations's position in relation to the basestation transmitters. The amount of transmissions time shift can thusvary from mobile station to mobile station.

According to a somewhat different, preferred embodiment, there isestimated in different base station receivers the reception time shiftbetween corresponding radio signals with the same message informationfrom the mobile station. This estimated reception time shift is utilizedfor selecting the transmission time shift between corresponding basestation transmitters. A transmission time shift can thus be individuallyobtained for each affected mobile station such that it is adjusted tothe position of this particular mobile station in relation to the basestation transmitters. The amount of transmission time shift can thusvary from mobile station to mobile station.

A preferred embodiment of a mobile, radio system in accordance with thepresent invention has a time measurement unit for estimating the mutualreception time shift in a mobile station between, on one hand, digitallymodulated radio signals transmitted to the mobile station via a basestation transmitter for the cell where this mobile station is, and, onthe other hand, corresponding digitally modulated signals transmitted tothis station via another base station transmitter for the cell. In thisembodiment the mobile radio system has a transmission time shifting unitfor mutually time shifting the transmission times for base stationtransmitters of the same cell so that they transmit correspondingdigitally modulated signals to the mobile station with greater or lessmutual transmission time shifting in response to estimated receptiontime shift.

The time measurement unit preferably includes an arrival time comparisonunit in at least certain mobile stations, for comparing the arrivaltimes for corresponding digitally modulated signals transmitted fromdifferent base station transmitters for the same cell.

According to a somewhat different, preferred embodiment, the timemeasurement unit includes an arrival time comparison unit in thestationary part of the mobile radio system, for comparing the arrivaltimes of digitally modulated signals transmitted from a mobile stationin a cell and received at different base station transceivers of thecell.

A method and a cellular digital mobile radio system in accordance withthe invention provide substantial advantages. The coverage degree can bemade greater, signifying better opportunities for establishing newconnections and maintaining those already established. The coveragedegree can be made greater without the cell size needing to be reduced,which give greater freedom in the selection of cell plan and a lessernumber of handovers. In preferred embodiments, the transmission timeshift, which is individually selectable for each mobile station, reducesthe risk of noise and interruption of calls in progress, when a mobilestation goes from receiving signals chiefly from one base stationtransmitter to receiving signals chiefly from another base stationtransmitter. When a mobile station simultaneously receives signals fromat least two base station transmitters of the same cell, the variabletransmission time shift enables the received signals together to be morelike the signals obtained from a single base station transmitter inconjunction with reflections.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates cells and positioning of transmitters in basestations in one embodiment of a mobile radio system in accordance withthe invention.

FIG. 2 illustrates parts of a mobile radio system in accordance with theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A cellular mobile radio system in accordance with the present inventionhas mobile stations and base stations with transmitters and receiversfor radio signals. Message information is transmitted digitally to andfrom the mobile stations by transmission and reception of signalsdigitally modulated in correspondance with the message information. Theradio signals are transmitted on one of a plurality of radio channels.Signals can be transmitted in time multiple to and from several mobilestations on the same radio channel.

The mobile stations move within and between the cells of the system. Thebase station transmitters are assigned to the cells so that there is atleast one base station transmitter for each cell, for transmittingsignals to the cell's mobile stations.

Somewhat simplified, there is illustrated in FIG. 1 the division of anarea into cells and the assignation of base station transmitters to thecells in a mobile telephone system in accordance with the presentinvention. For the sake of simplicity in FIG. 1, all cells C1 to C24 areillustrated as regular hexagons with sides L. In practice, the cellswill probably have different sizes and shapes. In addition, depending onservice conditions, it will often be suitable with overlapping in theboundary areas between the cells. To a certain extent, the base stationtransmitter can then off-load each other by handling transmissions tomobile stations where such transmissions should, from a purelygeographic point of view, be performed by the base stations of acontiguous cell.

For each cell C1-C24 there is an ordinary base station transmitterBS1-BS24. For contiguous cells these transmitters are conventionallyco-located in groups of three. For example, the base station transmitterBS1 for the cell C1 is co-located with the base station transmitter BS3for the cell C3 and the base station transmitter BS5 for the cell C5.Correspondingly, the base station transmitter BS14 for the cell C14 isco-located with the base station transmitter BS16 for the cell C16 andwith the base station transmitter BS18 for the cell C18. Theseco-positioned ordinary base station transmitters are situated in theboundary areas between the cells to which they are assigned. Forexample, the ordinary base station transmitters BS2, BS4 and BS6 areco-located in the boundary areas between the cells C2, C4 and C6.

Further to the ordinary base station transmitters BS1-BS24, the systemincludes a number of extra base station transmitters for certain of thecells. Cells C6, C7, C10, C11, C13, C14, C19 and C20 each has one extrabase station transmitter. For each of the cells C15, C18 and C22 thereare two extra base station transmitters. The extra base stationtransmitters XS6, XS7, XS10, XS11, XS13, XS15A, XS15B, XS18A, XS18B,XS19, XS20 and XS22A are co-located in groups with three extra basestation transmitters in each group in a similar manner as the ordinarybase station transmitters. Accordingly, for example, the extra basestation transmitter XS15B for the cell C15 is co-located with the extrabase station transmitter XS19 for the cell C19 and the extra basestation transmitter XS18A for the cell C18. On the other hand, neitherthe extra base station transmitter XS14 for the cell C14 nor the extrabase station transmitter XS22B for the cell C22 are co-located with anyother base station transmitter, but are situated approximately at thecenter of the cell with which they are associated.

An extra base station transmitter does not need to differ technicallyfrom an ordinary base station transmitter. For a given cell or extrabase station transmitter can thus have technical equipment of the sametype as an ordinary base station transmitter for the same cell. Inprinciple it can also function in the same way as the ordinary one. Ifthere are two identical base station transmitters for a given cell, incertain cases either of them may be respectively regarded as ordinary orextra.

The extra base station transmitter or transmitters for a given celltransmit radio signals which are substantially the same as those sent bythe ordinary base station transmitter of the cell. The radio signals aredigitally modulated with digital message information to the mobilestations in the cell. A mobile station in a cell for which there is oneor more extra base station transmitters can therefore receive, at leastin certain cases, corresponding radio signals from more than one basestation transmitter approximately simultaneously, within the samefrequency range (radio channel). Depending on the mutual, relativepositions in the cell of the mobile and base stations as well as thetransmission times and propagation paths of the radio signals from thebase station transmitters to the mobile stations, corresponding radiosignals from different base station transmitters can be receivedwithout, or with a given time shift at the mobile station. The greaterthe distance between the base station transmitters associated with thecell, the greater, in general can be the time shift. When the ordinarybase station transmitters and the extra base station transmitters aresituated according to FIG. 1, the distance between two base stationtransmitters for the same cell varies between L and 2L, i.e. between theside and diameter of the regular hexagons. If, for the sake ofsimplicity, reflections are ignored and the assumption made that thetransmitters transmit without mutual time shifting, the time receptionshift at the mobile station could then attain a maximum of 2L/c, where cis the propagation speed of the radio signals. C is approximately 300000km/s.

In FIG. 2 there are illustrated parts of a mobile radio system inaccordance present invention. A mobile radio exchange MSC is connectedvia cables L₁, L₂, . . . L_(m), L_(n) to a plurality of base stations,of which two, B_(m) and B_(n) are illustrated in FIG. 2.

The base station B_(rn) has a central unit connected via cables L_(ma)and L_(mb) to two transceiver units B_(ma) and B_(mb) situated at adistance from the central unit. The central unit of the base stationB_(m) includes a central line and control unit 1, transmission timeshifting (Tts) units 2A and 2B, one for each of the transceivers,reception time shifting (Rts) units 3A and 3B, one for each of thetransceivers and line units 4A and 4B, one for each of the transceivers.

Both transceivers in the base station B_(m) are alike. Each suchtransceiver contains a line and control unit (CU) 5A or 5B, transmitter(T) units 6A or 6B, receiver (R) units 7A or 7B, atransmission-reception filter (tRF) 8A or 8B and an antenna 9A or 9B.

The base station B_(n) differs partly from station B_(m), primarily dueto its central line and control unit (CU) 10 being situated inconnection with one of its transceivers B_(na). Accordingly, no cablewith associated line units corresponding to L_(m)L_(ma), L_(mb), 4A-5Bis needed for the transceiver B_(na), but only to the other transceiverB_(nb). In addition, no transmission or reception time shifting unitsare included in any central unit in B_(n), but the corresponding units2A, 2B, 3A and 3B are respectively included in transceivers B_(na) andB_(nb).

The mobile stations MS₁ and MS₂ are mutually alike. Each mobile stationincludes sound sensing (SS) unit 11, encoding (EN) unit 12, transmitting(T) unit 13, transmit-receive switch (TRS) 14, reception (A) unit 15,equalizer and decoding (ED) unit 16, sound reproducing (SR) unit 17,control unit (CU) 18 and a unit (ED) 19 for feeding in our out orpresentation of digital information.

Apart from the base stations having two transceiver units at a distancefrom each other, and having controllable transmission and reception timeshifting units, the mobile radio system in FIG. 2 functions in mostrespects in a way well-known in cellular mobile radio systems. Nocomplete description of how the system functions in different respectsshould therefore be necessary for one skilled in the art, and it shouldonly be necessary to describe what is unique or unusual in the cellularmobile radio system according to FIG. 2. One not skilled in the art ofcellular mobile radio systems is referred to the technical literatureand to the publications mentioned under the section “Background of theInvention”.

Message information that the mobile radio exchange forwards to a mobilestation in the cell C_(m), e.g. the station MS₁, is transmitted from themobile radio exchange via the cable L_(m) to the line and control unit1. From the line and control unit 1 the information is transferred viathe transmission time shifting unit 2A, line unit 4A, cable L_(ma) andline and control unit 5A to the transmitting unit 6A. The transmittingunit transmits via the transmission reception filter 8A and antenna 9Aradio signals with digit modulation in correspondance with the messageinformation from the mobile radio exchange.

The message information from the mobile radio exchange is alsotransferred from the line and control unit 1 via the transmission timeshifting unit 2B, line unit 4B, cable L_(mb) and line and control unit5B to the transmission unit 6B in the transceiver B_(mb). Thetransmission unit 6B transmits via the transmission reception filter 8Band antenna 9B radio signals with digital modulation in correspondancewith the information from the mobile radio exchange.

Depending on the delay in transferring the message information to thetransmission unit 6A and the corresponding delay in transferring to thetransmission unit 6B the radio signals can be transmitted from theantenna 9A of the transceiver unit B_(ma) substantially without timeshifting, or time shifted in relation to the transmission ofcorresponding radio signals from the antenna 9B of the other transceiverB_(mb).

The signals from the antenna 9A in B_(ma) arrive at a given mobilestation in the cell C_(m), e.g. the mobile station MS₁, with or withouttime shift in relation to corresponding radio signals from the antenna9B in B_(mb). The possible time shift on arrival at the mobile stationdepends partly on possible time shifting at transmission from theantennas and partly on possible difference in propagation time for theradio waves from the antennas. The transmission time shifting units 2Aand 2B have a variable delay and can be controlled by the line andcontrol unit 1, such that the radio signals are transmitted from theantenna 9A in B_(ma) time-shifted more or less before or aftercorresponding radio signals from the antenna 9B in B_(mb). In thepreferred embodiment according to FIG. 2, the line and control unit 1controls the variable delays in the transmission time shifting units 2Aand 2B so that the differences in delay in the cables L_(ma) and L_(mb)as well as the differences in the radio signal propagation times arecounteracted. This may also be expressed by saying that the line andcontrol unit controls the variables delays in the transmission timeshifting units 2A and 2B, such that the time shift of the radio waves onarrival at the mobile station is decreased compared if the units 2A and2B had the same fixed delay. It could be thought that the ideal casewere that the line and control unit controlled the delays in the timeshifting means 2A and 2B so that the digitally modulated radio signalstransmitted from the antenna in B_(ma) arrived at the antenna in MS₁,exactly simultaneously and in phase with corresponding radio signalstransmitted from the antenna in B_(mb). In practice, this is neitherdesired for nor achieved. Reflections occur in the propagation of theradio signals between the antennas, and the mobile station has anadaptive equalizer. It is therefore not necessary for the signals fromthe different transceiver units to arrive exactly simultaneously to themobile station. On the contrary, there is preferably sought a small timeshift to achieve diversity against Rayleigh fading. One not skilled inthis art and who is desirious of obtaining further information can findit in the publications mentioned in the section “Background Art”, e.g.“Radio Test Performance of a Narrow-band TDMA System-DMS 90”.

In principle, there are at least two conceivable methods of determininghow the line and control unit 1 may control the delay in thetransmission time shifting units 2A and 2B. One method is to estimate inthe fixed part of the mobile radio system the time shift between themobile station radio signals at one of the transceivers B_(ma) and thecorresponding radio signals at the other transceiver B_(mb). There isthus obtained an estimation of the differences in propagation time tothe mobile station, these differences depending on the position of themobile station. Remaining differences in delay are related to the fixedpart of the mobile radio system, e.g. differences in length of thecables L_(ma) and L_(mb) and are not dependent on the position of themobile station. In the embodiment according to FIG. 2, this method canbe applied in practice such that the delays in the reception timeshifting units 3A and 3B are adjusted so that information received fromthe mobile station MS₁ at B_(ma) arrives at the line and control unit 1.The delays in the transmission time shifting units 2A and 2B aresubsequently adjusted in correspondence with the optimum delays in thereception time shifting units 3A and 3B.

The other method is to estimate in the mobile station the difference inarrival time or time shift between the digitally modulated radio signalsfrom one transceiver Bma and the corresponding digitally mobile radiosignals from the other transceiver B_(mb). Some kind of encoding of theradio signals is required for this, which indicates from whichtransceiver they are transmitted. In TDMA systems it is known totransmit special synchronizing words. These can be utilized if they areformed or supplemented so that two base station transmitters for thesame cell do not only have identical synchronizing words. Alternatively,radio signals digitally modulated with special synchronising words canbe transmitted from the base station transmitters solely to enable themobile station to estimate the differences in arrival times or timeshifts. The mobile station transmits information about the estimatedarrival time difference or time shift via radio signals to the fixedpart of the system, where it is utilized for controlling thetransmission time shifting units 2A and 2B. The line and control unit 1then receives, via the respective line units 5A, 4A and 5B, 4B,information about estimated arrival time difference from the mobilestation in the same way as the line and control unit obtains messageinformation from the mobile station.

It is conceivable per se, but hardly to be preferred, to combine bothmethods for controlling the transmission time shift in a mobile radiosystem according to FIG. 2.

Measuring the difference in arrival time or time shifting forcorresponding radio signals can be performed in a conventional way, e.g.with the aid of correlation. In the cases where the radio signalsconventionally contain predetermined synchronizing patterns (words), thetime difference between these patterns (words) in different signals canbe measured using conventional methods. A mobile station control unit 18and/or a base station line and control unit 1, 10 in a possiblecombination with the transceivers' line units 5A and 5B can thencomprise time measurement means for estimating reception time shiftingor the arrival time comparison units for comparing arrival times.

When so required, a base station preferably utilizes conventionally thesame transmitter units and antenna for transmitting, in time multiplexwithin the same frequency range on the same radio channel, radio signalsdigitally modulated with message information to different mobilestations associated with the same cell. The radio signals withinformation to a given mobile station are then transmitted fromdifferent base station transmitters with a possible transmission timeshift which is specially adjusted with regard to the position of thisparticular mobile station. The case can arise where a base station in amobile radio system needs to transmit a radio signal with informationintended for reception by several or all of the mobile stations in thecell, e.g. information as to the identity of the base station/cell. Suchsignals are preferably transmitted without mutual time shifting from thetransceivers B_(ma), B_(mb) and B_(na), B_(nb) of the base stations in amobile radio system according to FIG. 2. The transmission time shiftingunits are then controlled to a balancing state where the delay ofinformation from the line and control unit 1 to the antenna in onetransceiver B_(ma) is equally as great as the delay of information fromthe line and control unit 1 to the antenna in the other transceiverB_(mb). The corresponding situation can apply when a base station“listens” in unoccupied combinations of time slot and frequency rangechannel for calls from mobile stations of unknown positions relative tothe transceivers of the base stations. The reception time shifting units3A and 3B can then be controlled to a balancing state where the delay ofthe message information from the antenna in one transceiver B_(na) tothe line and control unit 10 is equally as great as the delay ofinformation from the antenna in the other transceiver B_(nb) to the lineand control unit 10.

The mobile stations MS₁ and MS₂ have adaptive equalizers, whereby thedigital modulation during a modulation time interval in the radiosignals transmitted from a base station transmitter can be reconstructedfrom signals received during a reception time interval. In knowncellular digital mobile radio systems with only one base stationtransmitter per cell, the reception time interval of the equalizers isdimensioned according to the dispersion on the radio channel, i.e.expected time shifts between corresponding signals from a single basestation transmitter due to multipath propagation and reflections.Because of the equalizer, not only the radio signal having the greatestamplitude or arriving first to the mobile station is utilized forreconstructing the digital modulation, but also other correspondingradio signals arriving with a time shift within the extent of theequalizer's reception time interval can be utilized. The mobile stationsin a system in accordance with the invention have equalizers which aredimensioned such that the reception time interval of the mobile stationin reconstruction of the digital modulation is greater than the time ittakes for signals to propagate a distance as long as the greatestdistance between two base station transmitters associated with the samecell within a restricted geographical area. With the base stationtransmitters placed according to FIG. 1, and the restricted area beingthe area composed of the cells C1 to C24, the mobile station equalizerswould thus be dimensioned for a reception inverval of reconstructionwhich is greater than 2L/c. Taking into account that there can bedispersion, and that reflections can extend the propagation time frombase station transmitter to a mobile station more than the extension ofthe propagation time from another base station transmitter for the samecell, the reception time interval of the mobile stations is preferablysubstantially greater than the time it takes for radio signals topropagate a distance which is just as great as the greatest distancebetween two base station transmitters associated with the same cellwithin the geographical area in question.

In prior art mobile radio systems it is known to have cells of smallsize in city areas or densely populated areas with many calls per hourand to have cells of large size in rural areas with few calls per hour.Outside the restricted geographical area composed of cells C1 to C24 inFIG. 1 there might be cells of greater size than that of C1 to C24.Occasionally such cells may have plural base stations transmitting thesame information for overcoming radio shadows from mountains. A mobileradio according to the present invention may be used outside therestricted area and in such a cell but the advantages conferred by thepresent invention can not be expected if the transmission distancebetween the transmitters assigned to such a cell is too great inrelation to the reception time interval.

In mobile radio systems in accordance with the present invention, themodulation time interval of the digital modulation of the signals can beof the same order of magnitude as the time it takes for radio signals topropagate a distance just as long as the greatest transmitting distancebetween two base station transmitters serving the same cell within aparticular area. Although the present invention affords greateradvantages, the smaller the modulation time interval is in relation tothis propagation time, and the present invention has its greatestimportance when the modulation time interval is less than the mentionedpropagation time, the present invention can mean substantial advantageseven when the modulation time interval is some few times greater thanthe mentioned propagation time.

It is conceivable to use different kinds of digital modulation in amobile radio system, whereby somewhat different relationships can existbetween information transmission rate and modulation time interval. Indigital modulation of the radio signals involving transmission of onesymbol at a time of a sequence of uncorrelated symbols, the modulationtime interval will be the time during which a single symbol is decisivefor the modulation. For example, if a sequence of binary symbolsindividually and one at a time determine the modulation, the modulationtime interval will be the time during which one symbol determines themodulation. This can also be expressed by saying that the modulationtime interval will be the inverted value of the transmission rate inbits. In digital modulation of the signals involving two or more at atime of a sequence of digital symbols being decisive for the modulationduring wholely or partly overlapping times, the modulation interval canbe the time when a preceeding, but not the nearest subsequent symbolaffects the modulation. For example, in digital modulation according toSE 8102802-9, FIGS. 1 and 2, a symbol affects the phase of a carrierwave during a time interval 3T. The nearest preceeding symbol alsoaffects the phase change of the carrier wave during a first part 2T ofthe time interval 3T. The nearest subsequent symbol also affects thechange of phase of the carrier wave during a last part of 2T of the timeinterval 3T. In this case the modulation time interval will be T, whichagrees with what is called the symbol time interval in SE 8102802-9. Ingeneral, it can be said that the modulation time interval is to beinterpreted as the interval in time between two successive changes inthe transmitted digital modulation.

A method and a cellular digital mobile radio system in accordance withthe present invention is not restricted to the described embodiments,and can be modified within the scope of the claims. For example, it isconceivable to co-locate at least parts of the equipment in an ordinarybase station transmitter, with parts of the equipment in the extra basestation transmitter for the same cell, providing that their antennas arespaced from each other. In an extreme case it is conceivable inprinciple to have all equipment except the antennas co-located in oneplace, e.g. in the vicinity of one of the antennas, and that the antennaare fed with radio frequency signals by cable from this position. By“base station transmitter” is there fore meant at least a transmitterantenna for radio signals and preferably more or fewer of the remainingmeans required in a base station. In a base station transmitter thereare preferably included at least means corresponding to 6A and 6Bincluded in a transceiver unit B_(ma) or B_(mb) in FIG. 2.

We claim:
 1. A cellular mobile radio system comprising a plurality ofmobile stations which are movable within and between a plurality ofcells and a plurality of associated base stations assigned to said cellsfor digital transmission of message information, each of said pluralityof base stations and mobile stations having associated therewith arespective set of a transmitter and a receiver so that said messageinformation in the form of radio signals with digital modulation can becommunicated therebetween, said radio signals being digitally modulatedwith said message information within modulation time intervals, each ofsaid receivers including means for reconstructing the digital modulationfrom corresponding radio signals received within a predeterminedreception time interval; said cells and base stations being associatedwith one another in a manner such that at least two base stationtransmitters are assigned to each of predetermined cells within alimited geographic area, said at least two base station transmittersbeing disposed at a predetermined transmitting distance from each otherand each operable to transmit segments of digitally modulated radiosignals virtually simultaneously and within the same frequency rangewith identical message information being transmitted by the other ofsaid at least two base station transmitters to mobile stations within acell to which both of said transmitters are assigned, the digitallymodulated radio signals from said at least two base station transmittershaving modulation time intervals which are shorter than the timerequired for radio signals to propagate a distance which is as long as agreatest transmitting distance between two base station transmittersassigned to one cell within said geographic area, wherein saidreconstructing means associated with each of said receivers in saidplurality of mobile stations operate to reconstruct digital modulationof corresponding radio signals received during a reception time intervalwhich is at least as long as a time required for radio signals topropagate a distance which is as long as said greatest transmittingdistance.
 2. A cellular mobile radio system as claimed in claim 1,further comprising: time measurement means for estimating mutualreception time shifting in a mobile station between digitally modulatedradio signals transmitted to said mobile station from a base stationtransmitter associated with one of said predetermined cells where saidmobile station is located, and corresponding digitally modulated radiosignals transmitted to said mobile station from another base stationtransmitter associated with said one of said predetermined cells.
 3. Acellular mobile radio system as claimed in claim 2, wherein said timemeasurement means includes arrival time comparison means associated withsaid mobile station for comparing arrival times for correspondingdigitally modulated radio signals transmitted from different basestation transmitters associated with said one of said predeterminedcells.
 4. A cellular mobile radio system as claimed in claim 2, whereinsaid time measurement means includes arrival time comparison meansassociated with one of said at least two base station transmitters forcomparing arrival times for digitally modulated radio signalstransmitted from a second mobile station in a second of saidpredetermined cells and received at different base station transceiversassociated with said second of said predetermined cells.
 5. A cellularmobile radio system as claimed in claim 2, further comprising:transmission time shifting means for mutually time shifting transmissiontimes of said at least two base station transmitters associated withsaid one of said predetermined cells such that said at least two basestation transmitters transmit corresponding digitally modulated radiosignals to said mobile station with greater or less mutual transmissiontime shifting in response to the estimated reception time shifting.
 6. Amethod in a cellular mobile radio system for digitally transmittingmessage information between a plurality of mobile radio stations and aplurality of base stations associated with respective cells of saidcellular mobile radio system, said message information being in the formof digitally modulated radio signals that are transmitted and receivedby transmitters and receivers respectively associated with each of saidplurality of base and mobile stations, said method comprising the stepsof: transmitting digitally modulated radio signals with identicalmessage information from at least two base stations associated with thesame cell, and located at a predetermined distance from one another, tomobile stations within said cell, the digitally modulated radio signalsthat are transmitted from one of said base stations having certainsegments which are transmitted virtually simultaneously and within thesame frequency range as digitally modulated radio signals transmittedfrom the other of said two base stations; digitally modulating saidradio signals at transmission of said message information withinmodulation time intervals, said modulation time intervals being shorterthan the time necessary for radio signals to propagate a distance whichcorresponds to the greatest transmitting distance between two basestations associated with a cell of said cellular mobile radio system;and reconstructing the digital modulation from corresponding radiosignals that are received at a mobile station during a reception timeinterval, said reception time interval being at least as long as thetime required for radio signals to propagate a distance whichcorresponds to the greatest transmitting distance between said two basestations associated with said cell.
 7. A method as claimed in claim 6,wherein said digitally modulated radio signals with identical messageinformation are transmitted from different base station transmittershaving mutually variable transmission time shifting capabilities to apredetermined mobile station, and further including the step ofselecting said mutually variable transmission time shifting so as tocounteract differences in propagation time for said digitally modulatedradio signals from said different base station transmitters to saidpredetermined mobile station.
 8. A method as claimed in claim 7 furthercomprising the steps of: estimating an arrival time shift between saiddigitally modulated radio signals with said identical messageinformation from said different base station transmitters at saidpredetermined mobile station; transmitting information relating to theestimated arrival time shift from said predetermined mobile station toat least one base station; and utilizing the estimated arrival timeshift for selecting transmission time shifting for at least one basestation transmitter.
 9. A method as claimed in claim 7, furthercomprising the steps of: estimating a reception time shift betweenreception by different base station receivers of corresponding radiosignals with identical message information from said predeterminedmobile station; and utilizing said reception time shift for selectingsaid transmission time shift for corresponding base stationtransmitters.
 10. A cellular mobile radio system for communicatingmessage information within a geographic area that is divided intocommunication cells, comprising: a plurality of base stations associatedwith said cells, some of said cells having at least two base stationsassociated therewith and located a distance from one another to transmitrespective radio signals into a cell, which signals are digitallyencoded with the same message information and are transmitted at thesame frequency and substantially simultaneously with one another, eachbase station including means for digitally encoding the radio signalswith message information, said encoding being carried out withmodulation time intervals which are no longer than the time required foraudio signals to propagate a distance corresponding to the greatesttransmitting distance between two base stations associated with one cellin said system; and a plurality of mobile stations each having means forreconstructing the digital encoding of plural corresponding radiosignals respectively received over the same frequency range during areception time interval from the two base stations associated with acell, which reception time interval is at least as long as the timerequired for radio signals to propagate a distance corresponding to thegreatest transmitting distance between two base stations associated witha cell.
 11. A cellular mobile radio system comprising a plurality ofmobile stations which are movable within and between a plurality ofcells and a plurality of associated base stations assigned to said cellsfor digital transmission of message information, each of said pluralityof base stations and mobile stations having associated therewith arespective set of a transmitter and a receiver so that said messageinformation in the form of radio signals with digital modulation can becommunicated therebetween, said radio signals being digitally modulatedwith said message information within modulation time intervals, each ofsaid receivers including means for reconstructing the digital modulationfrom corresponding radio signals received within a predeterminedreception time interval; said cells and base stations being associatedwith one another in a manner such that at least two base stationtransmitters are assigned to each of predetermined cells within alimited geographic area, said at least two base station transmittersbeing disposed at a predetermined transmitting distance from each otherand each operable to transmit segments of digitally modulated radiosignals virtually simultaneously and within the same frequency rangewith identical message information being transmitted by the other ofsaid at least two base station transmitters to mobile stations within acell to which both of said transmitters are assigned, the digitallymodulated radio signals from said at least two base station transmittershaving modulation time intervals which are shorter than the timerequired for radio signals to propagate a distance which is as long as agreatest transmitting distance between two base stations transmittersassigned to one cell within said geographic area, wherein saidreconstructing means associated with each of said receivers in saidplurality of mobile stations operate to reconstruct digital modulationof corresponding radio signals received during a reception time intervalwhich is at least as long as a time required for radio signals topropagate a distance which is as long as said greatest transmittingdistance, and means for handing off a mobile station from one of said atleast two base stations to the other of said at least two base stations.12. A cellular mobile radio system comprising a plurality of mobilestations which are movable within and between a plurality of cells and aplurality of associated base stations assigned to said cells for digitaltransmission of message information, each of said plurality of basestations and mobile stations having associated therewith a respectiveset of a transmitter and a receiver so that said message information inthe form of radio signals with digital modulation can be communicatedtherebetween, said radio signals being digitally modulated with saidmessage information within modulation time intervals, each of saidreceivers including means for reconstructing the digital modulation fromcorresponding radio signals received within a predetermined receptiontime interval; said cells and base stations being associated with oneanother in a manner such that at least two base station transmitters areassigned to each of predetermined cells within a limited geographicarea, said at least two base station transmitters being disposed at apredetermined transmitting distance from each other and each operableradio transmit segments of digitally modulated radio signals virtuallysimultaneously and within the same frequency range with identicalmessage information being transmitted by the other of said at least twobase station transmitters to mobile stations within a cell to which bothof said transmitters are assigned, the digitally modulated radio signalsfrom said at least two base station transmitters having modulation timeintervals which are shorter than the time required for radio signals topropagate a distance which is as long as a greatest transmittingdistance between two base stations transmitters assigned to one cellwithin said geographic area, wherein said reconstructing meansassociated with each of said receivers in said plurality of mobilestations operate to reconstruct digital modulation of correspondingradio signals received during a reception time internal which is atleast as long as a time required for radio signals to propagate adistance which is as long as said greatest transmitting distance; andmeans for terminating a radio signal from one of said at least two basestations.
 13. A method in a cellular mobile radio system for digitallytransmitting message information between a plurality of mobile radiostations and a plurality of base stations associated with respectivecells of said cellular mobile radio system, said message informationbeing in the form of digitally modulated radio signals that aretransmitted and received by transmitters and receivers respectivelyassociated with each of said plurality of base and mobile stations, saidmethod comprising the steps of: transmitting digitally modulated radiosignals with identical message information from at least two basestations associated with the same cell, and located at a predetermineddistance from one another, to mobile stations within said cell, thedigitally modulated radio signals that are transmitted from one of saidbase stations having certain segments which are transmitted virtuallysimultaneously and within the same frequency range as digitallymodulated radio signals transmitted from the other of said two basestations; digitally modulating said radio signals at transmission ofsaid message information within modulation time intervals, saidmodulation time intervals being shorter than the time necessary forradio signals to propagate a distance which corresponds to the greatesttransmitting distance between two base stations associated with a cellof said cellular mobile radio system; reconstructing the digitalmodulation from corresponding radio signals that are received at amobile station during a reception time in said reception time intervalbeing at least as long as the time required for radio signals topropagate a distance which corresponds to the greatest transmittingdistance between said two base stations associated with said cell; andhanding off a mobile station from one of said at least two base stationsto another of said at least two base stations.
 14. A method in acellular mobile radio system for digitally transmitting messageinformation between a plurality of mobile radio stations and a pluralityof base stations associated with respective cells of said cellularmobile radio system, said message information being in the form ofdigitally modulated radio signals that are transmitted and received bytransmitters and receivers respectively associated with each of saidplurality of base and mobile stations, said method comprising the stepsof: transmitting digitally modulated radio signals with identicalmessage information from at least two base stations associated with thesame cell, and located at a predetermined distance from one another, tomobile stations within said cell, the digitally modulated radio signalsthat are transmitted from one of said base stations having certainsegments which are transmitted virtually simultaneously and within thesame frequency range as digitally modulated radio signals transmittedfrom the other of said two base stations; digitally modulating saidradio signals at transmission of said message information withinmodulation time intervals, said modulation the intervals being shorterthan the time necessary for radio signals to propagate a distance whichcorresponds to the greatest transmitting distance between two basestations associated with a cell of said cellular mobile radio system;reconstructing the digital modulation from corresponding radio signalsthat are received at a mobile station during a reception time interval,said reception time interval being at least as long as the time requiredfor radio signals to propagate a distance which corresponds to thegreatest transmitting distance between said two base stations associatedwith said cell; and terminating a radio signal from one of said at leasttwo base stations.
 15. A cellular mobile radio system for communicatingmessage information within a geographic area that is divided intocommunication cells, comprising: a plurality of base stations associatedwith said cells, at least one of said cells having at least two basestations associated therewith and located a distance from one another totransmit respective radio signals into said at least one cell, whichsignals are digitally modulated with substantially the same messageinformation and are transmitted at the same frequency and substantiallysimultaneously with one another, each base station including means fordigitally modulating the radio signals with message information, saidmodulating being carried out with modulation time intervals which arewithin a time interval related to the time required for radio signals topropagate a distance corresponding to the greatest transmitting distancebetween said at least two base stations associated with said at leastone cell in said system; and a plurality of mobile stations each havingmeans for reconstructing the digital modulation of plural correspondingradio signals respectively received over the same frequency range duringa reception time interval from said at least two base stationsassociated with said at least one cell, which reception time interval isat least as long as the time required for radio signals to propagate adistance corresponding to the greatest transmitting distance betweensaid at least two base stations associated with said at least one cell,wherein a mobile station is handed off from one of said at least twobase stations to another of said at least two base stations.
 16. Acellular mobile radio system for communicating message informationwithin a geographic area that is divided into communication cells,comprising: a plurality of base stations associated with said cells, atleast one of said cells having at least two base stations associatedtherewith and located a distance from one another to transmit respectiveradio signals into said at least one cell, which signals are digitallymodulated with substantially the same message information and aretransmitted at the same frequency and substantially simultaneously withone another, each base station including means for digitally modulatingthe radio signals with message information, said modulating beingcarried out with modulation time intervals which are within a timeinterval related to the time required for radio signals to propagate adistance corresponding to the greatest transmitting distance betweensaid at least two base stations associated with said at least one cellin said system; and a plurality of mobile stations each having means forreconstructing the digital modulation of plural corresponding radiosignals respectively received over the same frequency range during areception time interval from said at least two base stations associatedwith said at least one cell, which reception time interval is at leastas long as the time required for radio signals to propagate a distancecorresponding to the greatest transmitting distance between said atleast two base stations associated with said at least one cell, whereina radio signal is terminated from one of said at least two basestations.
 17. A cellular mobile radio system for communicating messageinformation within a geographic area that is divided into communicationcells, comprising: a plurality of base stations for transmitting radiosignals into a cell, which signals are digitally modulated withsubstantially the same message information and are transmitted at thesame frequency and substantially simultaneously with one another, eachbase station including means for digitally modulating the radio signalswith message information, said modulating being carried out withmodulation time intervals, which are at most a few times greater than atime required for radio signals to propagate a distance corresponding toa diameter of said cell; and a plurality of mobile stations each havingmeans for reconstructing the digital modulation of plural correspondingradio signals respectively received over the same frequency during areception time internal from the plurality of base stations, saidreception time interval is at least as long as the time required forradio signals to propagate a distance corresponding to said diameter ofsaid cell, wherein a mobile station is handed off from one of saidplurality of base stations to another of said plurality of basestations.
 18. A cellular mobile radio system for communicationinformation within a geographic area that is divided into communicationcells, comprising: a plurality of base stations for transmitting radiosignals into a cell, which signals are digitally modulated withsubstantially the same message information and are transmitted at thesame frequency and substantially simultaneously with one another, eachbase station including means for digitally modulating the radio signalswith message information, said modulating being carried out withmodulation time intervals which are at most a few times greater than atime required for radio signals to propagate a distance corresponding toa diameter of said cell; and a plurality of mobile stations each havingmeans for reconstructing the digital modulation of plural correspondingradio signals respectively received over the same frequency during areception time interval from the plurality of base stations, saidreception time interval is at least as long as the tune required forradio signals to propagate a distance corresponding to said diameter ofsaid cell, wherein a radio signal transmitted from one of said pluralityof base stations is terminated.
 19. A cellular mobile radio system forcommunicating message information and having a plurality of cells,comprising: a first base station for transmitting a first signal havingmessage information into a cell, a second base station for transmittinga second signal having said message information into said cell, a mobilestation in said cell having means for receiving said first and secondsignals, wherein said first and second signals are received with apropagation delay therebetween, wherein both of said first and secondbase stations include means for modulating said first and secondsignals, respectively, with said message information using a modulationtime interval which is no longer than a few multiples of saidpropagation delay, wherein said mobile station includes means forreconstructing said first and second signals during a reception timeinterval which is at least as long as said propagation delay, and meansfor handing off said mobile station from one of said base stations tothe other base station.
 20. A cellular mobile radio system forcommunicating message information and having a plurality of cells,comprising: a first base station for transmitting a first signal havingmessage information into a cell, a second base station for transmittinga second signal having said message information into said cell, a mobilestation in said cell having means for receiving said first and secondsignals, wherein said first and second signals are received with apropagation delay therebetween, wherein both of said first and secondbase stations include means for modulating said first and secondsignals, respectively, with said message information using a modulationtime interval which is no longer than a few multiples of saidpropagation delay, wherein said mobile station includes means forreconstructing said first and second signals during a reception timeinterval which is at least as long as said propagation delay, and meansfor terminating a radio signal from one of said base stations.
 21. Acellular mobile radio system for communicating message informationacross an area of coverage, comprising: a plurality of cells, each ofsaid plurality of cells representing a geographic division of said areaof coverage; a first base station for transmitting a first signalincluding message information into at least one of said plurality ofcells, said first base station including means for modulating a radiocarrier with said message information, said message information beingrepresented by a sequence of symbols; a second base station fortransmitting a second signal, including substantially the same messageinformation as transmitted by said first base station, into said atleast one of said plurality of cells, said second base station includingmeans for modulating said radio carrier frequency with saidsubstantially the same message information; and at least one mobilestation located within said at least one of said plurality of said cellswherein said first and said second signals are received by said mobilestation with a time shift therebetween wherein said time shift arisesfrom a difference in a first radio propagation delay between said atleast one mobile station and said first base station and a secondpropagation delay between said at least one mobile station and saidsecond base station, said at least one mobile station further includingmeans for recovering said message information from said first and saidsecond signals during a reception time interval which reception timeinterval is greater than said time shift, wherein a mobile station ishanded off from one of said base stations to the other base station. 22.A cellular mobile radio system for communicating message informationacross an area of coverage, comprising: a plurality of cells, each ofsaid plurality of cells representing a geographic division of said areaof coverage; a first base station for transmitting a first signalincluding message information into at least one of said plurality ofcells, said first base station including means for modulating a radiocarrier with said message information, said message information beingrepresented by a sequence of symbols; a second base station fortransmitting a second signal, including substantially the same messageinformation as transmitted by said first base station, into said atleast one of said plurality of cells, said second base station includingmeans for modulating said radio carrier frequency with saidsubstantially the same message information; and at least one mobilestation located within said at least one of said plurality of said cellswherein said first and said second signals are received by said mobilestation with a time shift therebetween wherein said time shift arisesfrom a difference in a first radio propagation delay between said atleast one mobile station and said first base station and a secondpropagation delay between said at least one mobile station and saidsecond base station, said at least one mobile station further includingmeans for recovering said message information from said first and saidsecond signals during a reception time interval which reception timeinterval is greater than said time shift, wherein a radio signal isterminated from one of said base stations.
 23. A cellular mobile radiosystem for communicating message information within a geographic areathat is divided into communication cells, comprising: a plurality ofbase stations associated with said cells, at least one of said cellshaving at least two base stations associated at a therewith and locateda distance from one another to transmit respective radio signals intosaid at least one cell, which signals are digitally modulated withsubstantially the same message information and are transmitted at thesame frequency and substantially simultaneously with one another, eachbase station including a transmitter that digitally modulates the radiosignals with message information, said modulation being carried out withmodulation time intervals which are within a time interval related tothe time required for radio signals to propagate a distancecorresponding to the greatest transmitting distance between said atleast two base stations associated with said at least one cell in saidsystem; a plurality of mobile stations each having a receiver thatreconstructs the digital modulation of plural corresponding radiosignals respectively received over the same frequency range during areception time interval from said at least two base stations associatedwith said at least one cell, which reception time interval is at leastas long as the time required for radio signals to propagate a distancecorresponding to the greatest transmitting distance between said atleast two base stations associated with said at least one cell; andmeans for handing off a mobile station from one of said at least twobase stations to another of said at least two base stations.
 24. Acellular mobile radio system for communicating message informationwithin a geographic area that is divided into communication cells,comprising: a plurality of base stations associated with said cells, atleast one of said cells having at least two base stations associatedtherewith and located a distance from one another to transmit respectiveradio signals into said at least one cell, which signals are digitallymodulated with substantially the same message information and aretransmitted at the same frequency and substantially simultaneously withone another, each base station including a transmitter that digitallymodulates the radio signals with message information, said modulationbeing carried out with modulation time intervals which are within a timeinterval related to the time required for radio signals to propagate adistance corresponding to the greatest transmitting distance betweensaid at least two base stations associated with said at least one cellin said system; a plurality of mobile stations each having, a receiverthat reconstructs the digital modulation of plural corresponding radiosignals respectively received over the same frequency range during areception time interval from said at least two base stations associatedwith said at least one cell, which reception time interval is at leastas long as the time required for radio signals to propagate a distancecorresponding to the greatest transmitting distance between said atleast two base stations associated with said at least one cell; andmeans for terminating a radio signal from one of said at least two basestations.
 25. A cellular mobile radio system for communicating messageinformation within a geographic area that is divided into communicationcells, comprising: a plurality of base stations for transmitting radiosignals into a cell, which signals are digitally modulated withsubstantially the same message information and are transmitted at thesame frequency and substantially simultaneously with one another, eachbase station including a transmitter that digitally modulates the radiosignals with message information, said modulation being carried out withmodulation time intervals which are at most a few times greater than atime required for radio signals to propagate a distance corresponding toa diameter of said cell; a plurality of mobile stations each having areceiver that reconstructs the digital modulation of pluralcorresponding radio signals respectively received over the samefrequency during a reception time interval from the plurality of basestation, said reception time interval is at least as long as the timerequired for radio signals to propagate a distance corresponding to saiddiameter of said cell; and means for handing off a mobile station fromone of said plurality of base stations to another of said plurality ofbase stations.
 26. A cellular mobile radio system for communicatingmessage information within a geographic area that is divided intocommunication cells, comprising: a plurality of base stations fortransmitting radio signals into a cell, which signals are digitallymodulated with substantially the same message information and aretransmitted at the same frequency and substantially simultaneously withone another, each base station including a transmitter that digitallymodulates the radio signals with message information, said modulationbeing carried out with modulation time intervals which are at most a fewtimes greater than a time required for radio signals to propagate adistance corresponding to a diameter of said cell; a plurality of mobilestations each having a receiver that reconstructs the digital modulationof plural corresponding radio signals respectively received over thesame frequency during a reception time interval from the plurality ofbase stations, said reception time interval is at least as long as thetime required for radio signals to propagate a distance corresponding tosaid diameter of said cell; means for terminating a radio signal fromone of said plurality of base stations.
 27. A cellular mobile radiosystem for communicating message information and having a plurality ofcells, comprising: a first base station for transmitting a first signalhaving message information into a cell, a second base station fortransmitting a second signal having said message information into saidcell, a mobile station in said cell having a receiver that receives saidfirst and second signals, wherein said first and second signals arereceived with a propagation delay therebetween, wherein both of saidfirst and second base stations include a transmitter that modulates saidfirst and second signals, respectively, with said message informationusing a modulation time interval which is no longer than a few multiplesof said propagation delay, wherein said mobile station receiverreconstructs said first and second signals during a reception timeinterval which is at least as long as said propagation delay, and meansfor handing off a mobile station from one of said base stations to theother of said base stations.
 28. A cellular mobile radio system forcommunicating message information and having a plurality of cells,comprising: a first base station for transmitting a first signal havingmessage information into a cell, a second base station for transmittinga second signal having said message information into said cell, a mobilestation in said cell having a receiver that receives said first andsecond signals, wherein said first and second signals are received witha propagation delay therebetween, wherein both of said first and secondbase stations include a transmitter that modulates said first and secondsignals, respectively, with said message information using a modulationtime interval which is no longer than few multiples of said propagationdelay, wherein said mobile station receiver reconstructs said first andsecond signals during a reception time interval which is at least aslong as said propagation delay, and means for terminating a radio signalfrom one of said base stations.
 29. A cellular mobile radio system forcommunicating message information across an area of coverage,comprising: a plurality of cells, each of said plurality of cellsrepresenting a geographic division of said area of coverage; a firstbase station for transmitting a first signal including messageinformation into at least one of said plurality of cells, said firstbase station including a transmitter that modulates a radio carrier withsaid message information, said message information being represented bya sequence of symbols; a second base station for transmitting a secondsignal, including substantially the same message information astransmitted by said first base station, into said at least one of saidplurality of cells, said second base station including a transmitterthat modulates said radio carrier frequency with said substantially thesame message information; at least one mobile station located withinsaid at least one of said plurality of said cells wherein said first andsaid second signals are received by said mobile station with a timeshift therebetween wherein said time shift arises from a difference in afirst radio propagation delay between said at least one mobile stationand said first base station and a second propagation delay between saidat least one mobile station and said second base station, said at leastone mobile station further including a receiver that recovers saidmessage information from said first and said second signals during areception time interval which reception time interval is greater thansaid time shift, and means for handing off a mobile station from one ofsaid base stations to the other of said base stations.
 30. A cellularmobile radio system for communicating message information across an areaof coverage, comprising: a plurality of cells each of said plurality ofcells representing a geographic division of said area of coverage; afirst base station for transmitting a first signal including messageinformation into at least one of said plurality of cells, said firstbase station including a transmitter that modulates a radio carrier withsaid message information, said message information being represented bya sequence of symbols; a second base station for transmitting a secondsignal, including substantially the same message information astransmitted by said first base station, into said at least one of saidplurality of cells, said second base station including a transmitterthat modulates said radio carrier frequency with said substantially thesame message information; at least one mobile station located withinsaid at least one of said plurality of said cells wherein said first andsaid second signals are received by said mobile station with a timeshift therebetween wherein said time shift arises from a difference in afirst radio propagation delay between said at least one mobile stationand said first base station and a second and propagation delay betweensaid at least one mobile station and said second base station, said atleast one mobile station further including a receiver that recovers saidmessage information from said first and said second signals during areception time interval which reception time interval is greater thansaid time shift, and means for terminating radio signal from one of saidbase stations.
 31. A cellular mobile radio system for communicatingmessage information and having a plurality of cells, comprising: a firstbase station for transmitting a first signal having message informationinto a cell, a second base station for transmitting a second signalhaving said message information into said cell, a mobile station in saidcell having a receiver that receives said first and second signals,wherein said first and second signals are received with a propagationdelay therebetween, wherein both of said first and second base stationsinclude a transmitter that modulates said first and second signals,respectively, with said message information using a modulation timeinterval which is no longer than a few multiples of said propagationdelay, wherein said mobile station receiver reconstructs said messageinformation from said first and second signals received during areception time interval which is at least as long as said propagationdelay, and wherein said first base station terminates transmission ofsaid first signal while said second base station continues to transmitsaid second signal to thereby handoff said mobile station from saidfirst base station to said second base station.
 32. A cellular mobileradio system for servicing calls to and from a plurality of mobilestations located within an area of coverage that is divided into aplurality of cells, said system providing for handover of a call inprogress to one of said mobile stations as said mobile station movesbetween adjacent cells, said system comprising: a first base station fortransmitting a first signal including message information into it leastone of said plurality of cells, said message information transmitted asa sequence of symbols representing a call to one of said mobilestations; a second base station for transmitting a second signal,including substantially the same message information as transmitted bysaid first base station, into said at least one of said plurality ofcells, said second base station commencing the simultaneous transmissionof substantially the same message information as said first base stationprior to said handover of said call from said first base station to saidsecond base station; and at least one mobile station located within saidat least one of said plurality of said cells wherein said first and saidsecond signals are received by said mobile station with a time shifttherebetween wherein said time shift arises from a difference in a firstradio propagation delay between said at least one mobile station andsaid second base station, said at least one mobile station furtherincluding a receiver that recovers said message information from acombination of said first and said second signals received during areception time interval which reception time interval is greater thansaid time shift, said system performing the handover of said call inprogress by terminating the transmission of said first signal when saidmobile station moves from said at least one of said plurality of cellsinto an adjacent cell.
 33. A cellular mobile radio system for servicingcalls to and from a plurality of mobile stations located within an areaof coverage that is divided into a plurality of cells, said systemproviding for handover of a call in progress to one of said plurality ofmobile stations as said one of said plurality of mobile stations movesbetween adjacent cells, said system comprising: a plurality of basestations associated with said plurality of cells for transmitting radiosignals into said cells, each base station including a transmitter thatis adapted to digitally modulate the radio signals with messageinformation, one of said radio signals being modulated with messageinformation representing said call in progress to said one of saidplurality of mobile stations, said modulation being carried out withmodulation time intervals which are no longer than a few multiples of apropagation delay associated with reception of said radio signals; andeach of said plurality of mobile stations having a receiver thatreconstructs the digital modulation of plural corresponding radiosignals respectively received during a reception time interval from theplurality of base stations, said reception time interval is at least aslong as said propagation delay, said system performing a handover ofsaid call in progress from a first one of said plurality of basestations to one of said plurality of mobile stations in one of saidplurality of cells by commencing the simultaneous transmission from asecond one of said plurality of base stations in an adjacent cell ofsubstantially the same message information as transmitted by said firstbase station and terminating the transmission of said messageinformation from said first base station when said mobile station hasmoved into said adjacent cell.
 34. A method for handing off a callbetween a mobile station and a first base station to a second basestation in a cellular mobile radio system comprising the steps of:providing a plurality of cells, each of said plurality of cellsrepresenting a geographic division of said area of coverage;transmitting, from said first base station, a first signal includingmessage information into at least one of said plurality of cells;transmitting, from said second base station, a second signal, includingsubstantially the same message information as said first signal, intosaid at least one of said plurality of cells; receiving, at said mobilestation, said first and second signals with a time shift therebetween,wherein said time shift arises from a difference in a first radiopropagation delay between said at least one mobile station and saidfirst base station and a second propagation delay between said at leastone mobile station and said second base station; combining, in saidmobile station, information from said first and said second signalsreceived during a reception time interval which reception time intervalis greater than said time shift; and terminating, at said first basestation, transmission of said first signal, thereby handing off saidcall to said second base station.